Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

precompiled-headers.h

@@ -42,19 +42,22 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
-#include <grp.h>
-#include <netdb.h>
-#include <pwd.h>
 #include <signal.h>
-#include <sys/resource.h>
-#include <sys/select.h>
-#include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
-#include <sys/utsname.h>
-#include <sys/wait.h>
-#include <termios.h>
 #include <unistd.h>
 
+#ifndef _WIN32
+# include <grp.h>
+# include <netdb.h>
+# include <pwd.h>
+# include <sys/resource.h>
+# include <sys/select.h>
+# include <sys/socket.h>
+# include <sys/utsname.h>
+# include <sys/wait.h>
+# include <termios.h>
+#endif
+
 #include <nlohmann/json.hpp>